It is often necessary to move individual pieces of a stack of materials, such as sheets of cloth or fiberboard, from one position to another, one at a time, for work to be performed thereon. Mechanisms to perform this operation generally do so by providing a gripper foot which pierces and penetrates the surface of the top sheet of material in order to hold, lift and move it. This clearly has potential for damaging material.
An example of this type of pick-up mechanism may be seen in Engelmann U.S. Pat. No. 3,176,979. As may be thereseen in FIG. 1, oppositely directed, angled rows of piercing needles 10 and 11 are supported so as to slidably reciprocate parallel to each other. When the needles engage a sheet of material, they are moved in opposite directions so as to pierce and stretch the sheet of material to hold it. To release the material, the rows of needles are moved back apart, thus exiting the material and dropping it. Such a mechanism has, of course, the potential for piercing more than one sheet of material if the needles are longer than the material is thick.
Another type of pick-up device may be seen in the Rovin U.S. Pat. No. 3,625,506. Rovin is a variation of the design of Engelmann which seeks to differentiate the top sheet of a stack of sheets before it is pierced. As may be seen in FIG. 13 thereof, stop surfaces 64 on gripper feet 62 mound up a patch 66a of sheet 66 prior to pointed feet 57 piercing the material. It would be desirable to provide a gripper mechanism which would not pierce the material, thus avoiding the possibility of damage thereto, while still differentiating and securely holding and differentiating the top sheet of a stack.